Coated electrochemical corrosion potential sensor

Abstract

An electrochemical corrosion potential sensor includes a sensor tip electrically joined to a conductor, and a ceramic insulator joined to the tip around the conductor. A sleeve is joined to the insulator around the conductor, and is electrically insulated from the tip by the ceramic insulator. The insulator has an exposed surface axially separating the tip and sleeve, and a ceramic coating is bonded thereto for preventing dissolution of the insulator by reactor water. In a preferred embodiment, the ceramic insulator is sapphire, and the ceramic coating is yttria-stabilized-zirconia or magnesia-stabilized-zirconia which may be plasma sprayed over the insulator.

What is claimed is: 
     
       1. A sensor for measuring electrochemical corrosion potential in a nuclear reactor comprising: a sensor tip electrically joined to a conductor;   a tubular ceramic insulator joined to said tip around said conductor;   a tubular sleeve joined to said ceramic insulator around said conductor, and electrically insulated from said tip by said ceramic insulator;   said ceramic insulator having an exposed surface axially separating said tip and sleeve; and   a ceramic coating bonded to said exposed surface and adjoining portions of said tip and sleeve for preventing dissolution of said ceramic insulator by circulating water in said reactor.   
     
     
       2. A sensor according to claim 1 wherein: said tip is a noble metal;   said sleeve is a metal;   said ceramic insulator is joined to said tip and sleeve at corresponding ceramic-to-metal braze joints; and   said coating extends over said braze joints.   
     
     
       3. A sensor according to claim 2 wherein: said ceramic insulator is sapphire; and   said coating is yttria-stabilized-zirconia or magnesia-stabilized-zirconia.   
     
     
       4. A sensor according to claim 2 further comprising: a bond coating bonded to said ceramic insulator, tip, and sleeve across said braze joints; and   said ceramic coating is in turn bonded atop said bond coating.   
     
     
       5. A sensor according to claim 4 wherein said ceramic coating is directly bonded in part to an intermediate portion of said ceramic insulator at said exposed surface, and bonded to said bond coating axially therefrom for maintaining electrical insulation between said tip and sleeve. 
     
     
       6. A method of fabricating the sensor of claim 1 for improving life thereof in said reactor comprising: roughening said exposed surface and adjoining surface portions of said tip and sleeve; and   plasma spraying said ceramic coating across said roughened surfaces to bond said ceramic coating thereto.   
     
     
       7. A method according to claim 6 wherein said plasma spraying is effected at a temperature less than a temperature for brazing said joints. 
     
     
       8. A method according to claim 7 further comprising: first depositing a bond coating on said ceramic insulator, tip, and sleeve, across said braze joints; and   secondly plasma spraying said ceramic coating atop said bond coat.   
     
     
       9. A method according to claim 8 further comprising: masking an intermediate portion of said ceramic insulator at said exposed surface prior to depositing said bond coating to maintain electrical insulation thereat; and   plasma spraying said ceramic coating atop said bond coating, and atop said intermediate portion directly atop said exposed surface.   
     
     
       10. A method according to claim 9 wherein: said ceramic insulator is sapphire; and   said coating is yttria-stabilized-zirconia or magnesia-stabilized-zirconia.